With DH cake mix, they generally make 5-6 cups of batter, but it varies on flavor. White mix usually makes the lower end, around 5, chocolate falls in the middle, and the only one I know of that makes (or did) a full 6 cups of batter is the Golden Butter Recipe.

As was mentioned using a recipe like the WASC or the extender recipes you can bulk up your cake mixes, and add about two cups more batter per mix.